Fix JIT infinite recompile loop for closure functions

When a JIT-compiled function failed on first call, the function name
was added to _jit_warned but this was never checked before recompiling.
Closures like parse-loop create new lambda instances on each call,
each with l_compiled=None, triggering fresh compilation + failure
in an infinite loop.

Fix: check _jit_warned before attempting compilation, and mark the
lambda with jit_failed_sentinel on first-call failure so the same
instance also stops retrying.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hosts/ocaml/bin/sx_server.ml

@@ -727,7 +727,7 @@ let register_jit_hook env =
             None)
        | Some _ -> None  (* compile failed or disabled — CEK handles *)
        | None ->
-         if !_jit_compiling then None
+         if !_jit_compiling || Hashtbl.mem _jit_warned fn_name then None
          else begin
            _jit_compiling := true;
            let t0 = Unix.gettimeofday () in
@@ -742,6 +742,7 @@ let register_jit_hook env =
              (try Some (Sx_vm.call_closure cl args cl.vm_env_ref)
               with e ->
                 Printf.eprintf "[jit] %s first-call fallback to CEK: %s\n%!" fn_name (Printexc.to_string e);
+                l.l_compiled <- Some Sx_vm.jit_failed_sentinel;
                 Hashtbl.replace _jit_warned fn_name true;
                 None)
            | None -> None